We are lucky that carrier doesnt associate "trip to vet" with our gang. They go in the carriers during tornado warnings, also they have traveled in them. RB Butterscotch loved the carrier when we had work done on the house, he could watch at a safe distance from his carrier.

I might suggest putting the carrier out in a room with door open and let them get used to it being in the room. They might even nap or play in the carrier.